Oscillator circuits are widely used in integrated circuits to accomplish various tasks, such as the generation of internal voltage references, charge pumping and other functions. A very commonly used MOS oscillator circuits is a ring oscillator which has an odd number of inverter stages connected in a positive feedback loop, as shown in FIG. 1. This circuit operates by the switching of each inverter from one logic state to another. Assuming initially that the output terminal, which had been at logic 1, switches to logic 0, then by the feedback loop, the output node of the first inverter switches from 0 to 1.
The switch from 0 to 1 at that output node occurs after an RC time constant, R being the resistive load at the output node and C being the capacitance at that node. Similarly, the output node of the second inverter switches from 1 to 0 logic states after the designed RC time constant. Therefore, after the sum of all of the RC time constants of the stages, typically 50-100 stages, the output terminal changes state from 0 to 1 again. This switches the first inverter again and this circuit switches back and forth so that at any one node, the logic state oscillates between 1 and 0.
A major drawback of this type of circuit is that it is highly dependent on the stability of the power supply to the oscillator circuit and minimal process variations used in forming the integrated circuit elements of the oscillator circuit. For example, a variation in the positive voltage supply, V.sub.CC, from +4.5 volts to +5.5 volts may cause the period of oscillation to vary as much as 100%. This is not acceptable for most applications.
On the other hand, the present invention provides for a MOS oscillator circuit which is substantially independent of power supply variations. Furthermore, the oscillator circuit uses typical MOS process device elements so that no variation in a MOS semiconductor process is required to manufacture the oscillator circuit.